As a wheel bearing apparatus for a part-time 4WD vehicle which can be switched between a two-wheel drive (2WD) mode and a four-wheel drive (4WD) mode, there has been known a structure which can selectively couple a drive shaft (an axle) connected to an engine to a wheel and cancel the coupling, that is, can selectively allow and shut off a transmission of a driving force from the drive shaft to the wheel (refer, for example, to Patent Documents 1 and 2).
In the wheel bearing apparatus mentioned above, a roller bearing for rotatably supporting an inner shaft (a hub shaft) with respect to a suspension apparatus is installed to an outer circumference of the inner shaft to which the wheel is attached. A center of the inner shaft is provided with a center hole to which a drive shaft is inserted. The drive shaft is rotatably supported to the inner shaft by a needle bearing press fitted into the center hole. Further, the wheel bearing apparatus is provided with a coupler ring receiving the transmission of the rotation from the drive shaft via a clutch member. The coupler ring is fixed to an end portion in a vehicle inner side (an opposite side to a side in which the wheel is attached) of the inner shaft, and has a spline in an outer circumferential surface. If the clutch member rotating in synchronous with the drive shaft is engaged with the outer circumferential spline of the coupler ring, the drive shaft and the inner shaft are operationally coupled, and the transmission of the driving force from the drive shaft to the wheel is allowed. On the other hand, if the clutch member is not engaged with the outer circumferential spline of the coupler ring, the operational coupling between the drive shaft and the inner shaft is cancelled, and the transmission of the driving force from the drive shaft to the wheel is shut off.
In the wheel bearing apparatus mentioned above, a spline is formed in an inner circumferential surface of the coupler ring and the inner side end portion of the inner shaft. The coupler ring and the inner shaft are coupled so as to be integrally rotatable, on the basis of the spline engagement between the coupler ring and the inner shaft. The coupler ring is fixed to the inner shaft in an axial direction by caulking the inner side end portion of the inner shaft and plastically deforming the end portion radially outward. The shaft end caulking mentioned above is executed by pressing a caulking jig against the inner side end portion of the inner shaft from the vehicle inner side in a state of bringing a vehicle outer side end surface of the inner ring of the roller bearing into contact with a flange portion side surface of the inner shaft, and bringing an end surface of the coupler ring into contact with the vehicle inner side end surface of the inner ring. Accordingly, the coupler ring and the inner ring are reliably fixed to the inner shaft, and a suitable preload is applied to the coupler ring and the inner ring. Since two wheels corresponding to coasting wheels at a time of traveling in the 2WD mode do not rotate a drive system (an axle shaft, a differential, a propeller shaft or the like) in the case of using the wheel bearing apparatus, it is possible to achieve an improvement of a fuel consumption, a reduction of a vibration and a noise, an improvement of a durability of driving parts.
In the wheel bearing apparatus mentioned above, the end surface of the coupler ring is pressed to the end surface of the inner ring of the roller bearing. Accordingly, there is a risk that the end surface of the coupler ring comes into contact with the seal member provided in the roller bearing. If a deformation is generated in the seal member on the basis of the contact of the end surface of the coupler ring, a function of the seal member for preventing a foreign material from making an intrusion into the inner portion of the bearing is lowered. Further, there is a case that a burr at a time of forming the spline is left in the end surface of the outer circumferential spline in the end surface of the coupler ring, in a state of being attached thereto. Accordingly, in the case that the end surface of the outer circumferential spline of the coupler ring comes into contact with the seal member, such an interference that the burr presses the seal member may be generated.
Further, in the wheel bearing apparatus mentioned above, the shaft end caulking is executed by pressing the caulking jig to the inner side end portion of the inner shaft from the vehicle inner side. Accordingly, a bulge in a radial direction is generated in the inner shaft on the basis of the shaft end caulking. The bulge mentioned above can be generated in an inner circumferential side in addition to an outer circumferential side of the inner shaft. If the bulge is generated in the inner circumferential side of the inner shaft, that is, an inner wall of the center hole is bulged, a needle bearing is deformed at a time of press fitting the needle bearing to the center hole so as to assemble. Alternatively, the needle bearing which is previously arranged within the center hole is deformed. In the case that an inscribed circle diameter of the needle bearing becomes smaller due to the deformation of the needle bearing, an overload in a radial direction is applied to the drive shaft. As a result, a rotation of the drive shaft is obstructed, thereby causing a deterioration of the fuel consumption and a deterioration of the durability of the needle bearing and the drive shaft.
Further, in the case of the shaft end caulking, it is not easy to accurately apply a preload having a desired magnitude to the bearing. This is because the pressure applied to the end portion of the inner shaft for caulking the end portion of the inner shaft is also dispersed to the radial direction of the inner shaft. Further, the reasons include the fact that the coupler ring is interposed between the caulked portion and the bearing. In other words, it becomes complicated to control the preload for accurately applying the preload having the desired magnitude to the bearing and a lot of manufacturing processes are required. Further, in order to further improve a steering feeling at a time of traveling the vehicle, there is required a further improvement of the rigidity in the wheel bearing apparatus.    Patent Document 1: Japanese National Phase Laid-Open Patent Publication No. 2003-507683    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-351336